Traditionally a telephone terminal apparatus, for example telephones and the like, is coupled through an associated telephone line with a central power source via a line interface circuit. The line interface circuit includes circuitry, usually of a resistive nature, for feeding energizing direct current from the central power source to the telephone terminal apparatus. Recently, various active line interface circuits have been developed wherein the function of feeding the energizing direct current is performed by active elements which may optimize the coupling of the telephone with an associated telephone system. Such active line circuits may include over current protection circuitry which responds to unintended operational faults, for example power line crosses, short circuits or ground faults, by somewhat limiting current flow in the interest of preventing catastrophic failure of line interface circuit.
Recently telephone terminal apparatus of a digital nature have been developed to take advantage of the recommended Integrated Services Digital Network (ISDN) standard. In ISDN jargon a terminal apparatus is usually referred to as a terminal equipment (TE), a line interface circuit as a network termination (NT1), and a line for connection between a NT1 and a TE as a terminal (T) interface, hereafter referred to as a T bus. The TEs are characterized by digital circuitry requiring an operating voltage or voltages not conventionally available from an associated telephone facility. However the traditional reliability of telephony service is never the less preferred. Hence one arrangement is provided wherein a convenient physical location for a group of NT1s is also provided with a line power source, which is intended to be more reliable than the supply service expected from a local electrical utility. Each of the NT1s is provided with power from the line power source, at a potential of about 50 volts, so that energizing direct current is made available to each associated TE via the wires of the interconnecting T bus. A typical TE includes a direct current to direct current (DC to DC converter which utilizes between about 40 to 60 milliamperes of current from the T bus to provide the required voltage or voltages for normal operation. However initiation of operation of a TE, such as when it is first plugged into a T bus or when power is initially applied at the NT1, typically draws a momentary surge of current. The surge of current is that which is required to initiate operation of the typical DC to DC converter and associated filter capacitors. In a normal power up event in a TE, the DC to DC converter usually draws a current peak or current inrush, which may exceed an ampere for as much as ten milliseconds. In such circumstances the typical current limiting circuit intended to protect the typical line interface circuit is inappropriate for operation throughout the whole current load regime. For example, if a current limit of twice the normal operating current is set, there will be insufficient current for start up of the DC to DC converter and on the other hand if a current limit sufficiently great to accommodate start up is set, a fault may be permitted to draw current for a period of time sufficient to seriously jeopardize the operations of the NT1 physically adjacent, and the line power source circuits.
It is an object of the invention to supply operating current from a central line power source via a line interface circuit, to a terminal equipment having a DC to DC converter, while providing an over current protection feature which is effective across the entire load current regime of the terminal equipment.